The embodiments disclosed relate generally to magnetic bearings for rotary machines having a rotor. In particular, the embodiments relate to magnetic bearings for rotary machines in which the rotor and the bearing are, in use, in contact with a fluid, for instance a gaseous atmosphere, that is corrosive, acid, or carrying particles. Some exemplary embodiments specifically relate to a rotary machine comprising such a magnetic bearing.
Use of magnetic bearings to rotary machines is becoming more and more widespread, in particular in case of corrosive fluid. When the operating fluid of the machine in which the bearing operates is either acid, or corrosive or carrying particles, it is essential to protect the coils of the magnetic bearing and the associated means, by using anti-corrosion protection technologies. An example of such a technology is the jacketed bearing in which the stator portion of the bearing is protected by a metal jacket made of a material that does not oxidize or corrode and in general does not suffer any phenomena related to the aggressiveness of the environment.
A jacket may be in the form of a plate welded to an annular support in which is placed a stator magnetic circuit comprising at least one coil and a ferromagnetic body. The annular support may be made of a stainless material, such as magnetic stainless steel. The jacket of closure plate may be made out of the same material as the annular support, or it may be of a different metallic material, such as nickel base alloy for example.
In order to withstand operating conditions (pressure, fast variations in pressure, temperature, ability to withstand corrosion and abrasion), the jacket generally presents thickness lying in the range of 0.3 to 1 mm or more, for instance in the range of 0.3 to 0.5 mm, i.e. similar to that of the airgap of the magnetic bearing (which is the distance between the stator magnetic circuit and a rotor armature of the bearing). The presence of such a jacket of non-magnetic material thus amounts to increasing the thickness of the airgap of the bearing, which leads to a significant limit on the load available from said bearing. Moreover, it does not totally ensure the lack of contacts between the jacket and the rotor armature of the magnetic bearing, in any conditions.
It is therefore desirable for the thickness of the jacket to be reduced and for the jacket to be constituted by a fine metal sheet. Nevertheless, it requires the use of specific materials having high mechanical properties and anti-corrosion properties, in order to ensure the protection of the stator magnetic circuit against corrosion and to keep the same shape and dimensions in operation.
Furthermore, the production of the jacket, after welding and final re-machining, does not allow to check the exact thickness of the jacket, and thus the actual airgap of the bearing.